Protests will be held on Monday as new fees of up to £1,200 come into force for workers taking employment tribunal cases against their employers.

Lawyers are predicting widespread chaos as charges are introduced for those pursuing sexual harassment or race discrimination complaints. Trade unions say the move – which ministers claim will save money for businesses and taxpayers – is the latest attack on workers' fundamental rights.

The TUC general secretary, Frances O'Grady, said: "Today is a great day for Britain's worst bosses. By charging upfront fees for harassment and abuse claims, the government is making it easier for employers to get away with the most appalling behaviour."

She said the initiative was just the latest move by the coalition to undermine people's rights in the workplace. "These reforms are part of a wider campaign to get rid of workers' basic rights at work. Its only achievement will be to price vulnerable people out of justice."

The government defended the fees, saying the change would remove the burden of speculative cases from businesses and taxpayers.

The justice minister Helen Grant said: "It is not fair on the taxpayer to foot the entire £74m bill for people to escalate workplace disputes to a tribunal. We want people, where they can afford to do so, to pay a contribution.

"It is in everyone's interest to avoid drawn out disputes which emotionally damage workers and financially damage businesses. That's why we are encouraging quicker, simpler and cheaper alternatives like mediation."

The government says waivers are available (pdf) for those who cannot afford the fees, and anyone who pays and goes on to win their case can apply to the judge to have their fees paid by the other side.

However, Len McCluskey, leader of the Unite union, said the introduction of fees was a "throwback to Victorian times".

"Seeking redress for unfair dismissal and discrimination and other injustices in the workplace is a fundamental human right – but now ministers are putting up insurmountable financial hurdles for working people in pursuit of justice," he said.

"We estimate that this will affect 150,000 workers a year. This is not an aid to economic recovery but a means to keep working people frightened and insecure."

Unite said it would pay legal fees for its members. McCluskey said: "Unite will not stand idly by and let its members suffer such treatment – and the union will financially support members at employment tribunals. We will be campaigning strongly for this pernicious legislation to be repealed with the advent of a Labour government."

The GMB union will stage a protest outside an employment tribunal in central London.

Elizabeth George, a barrister in the employment team at law firm Leigh Day, said the fees would hit some of the most vulnerable people in the workplace, including new mothers returning to work after maternity leave.

"This sends a very dangerous message to employers who will be less inclined to abide by their legal obligations as the risk of being challenged will be much reduced," she said.

"These fees will disproportionately hit those suffering discrimination because of their age, race, disability and gender, with women returning from maternity leave particularly hard hit as they'll be judged on their salary when they left rather than their statutory maternity pay."

She said the introduction of "substantial and often disproportionate fees" would mean many people having to put up with discrimination and unable to challenge unfair dismissals, however badly they had been treated.

"We have already seen guidance from lawyers advising employers to wait to fire people as it will be cheaper and the chances of being taken to tribunal will be less following the introduction of fees. Instead of standing up for people suffering unlawful discrimination in the workplace, the government is doing the exact opposite by potentially penalising those who get pregnant, those who are disabled, those who fall ill and those who grow old."